add output-list and output-get

This patch adds the "output-list" and "output-get" commands to heat.
The first outputs a list of output_keys on stdout, and the second prints
a single output_value to stdout.

For non-string values, dumps the value in JSON.

This patch includes tests for the new commands as well as updated
documentation.

Change-Id: Iaade16d043b2c42d14f642fde17419eaa07d0ab8
Closes-Bug: 1258622